A plasma is the term used for an electrically conductive gas consisting of positive and negative ions, electrons and excited and neutral atoms and molecules which is heated thermalbly to a high temperature.
Various gases are used as plasma gases, such as mono-atomic argon and/or the diatomic gases hydrogen, nitrogen, oxygen or air. These gases are ionised and dissociated by the energy of an electric arc. The electric arc is constricted by a nozzle and is then referred to as a plasma jet.
The parameters of the plasma jet can be heavily influenced by the design of the nozzle and the electrode. These parameters of the plasma jet are, for example, the diameter of the jet, the temperature, the energy density and the flow rate of the gas.
In plasma cutting, for example, the plasma is constricted by a nozzle, which can be cooled by gas or water. In this way, energy densities of up to 2×106 W/cm2 can be obtained. Temperatures of up to 30,000° C. arise in the plasma jet, which, in combination with the high flow rate of the gas, make it possible to achieve very high cutting speeds on materials.
Plasma burners can be operated directly or indirectly. In the direct operating mode, the current flows from the source of the current, through the electrode of the plasma burner and the plasma jet generated by the electric arc and constricted by the nozzle, directly back to the source of the current via the workpiece. The direct operating mode can be used to cut electrically conductive materials.
In the indirect operating mode, the current flows from the current source, through the electrode of the plasma burner and the plasma jet generated by the electric arc and constricted by the nozzle, and back to the source of the current via the nozzle. In the process, the nozzle is subjected to an even greater load than in direct plasma cutting, since it not only constricts the plasma jet, but also establishes the attachment spot for the electric arc. With the indirect operating mode, both electrically conductive and non-conductive materials can be cut.
Because of the high thermal stress on the nozzle, it is usually made from a metallic material, preferably copper, because of its high electrical conductivity and thermal conductivity. The same is true of the electrode holder, though it may also be made of silver. The nozzle is then inserted in a plasma burner, the main elements of which are a plasma burner head, a nozzle cap, a plasma gas conducting member, a nozzle, a nozzle holder, an electrode quill, an electrode holder with an electrode insert and, in modern plasma burners, a bracket for a nozzle protection cap and a nozzle protection cap. The electrode holder fixes a pointed electrode insert made from tungsten, which is suitable when non-oxidising gases are used as the plasma gas, such as a mixture of argon and hydrogen. A flat-tip electrode, the electrode insert of which is made of hafnium, is also suitable when oxidising gases are used as the plasma gas, such as air or oxygen. In order to achieve a long service life for the nozzle, it is in this case cooled with a fluid, such as water. The coolant is delivered to the nozzle via a water supply line and removed from the nozzle via a water return line and in the process flows through a coolant chamber, which is delimited by the nozzle and the nozzle cap.
DD 36014 B1 describes a nozzle. It consists of a material with good conductive properties, such as copper, and has a geometrical shape associated with the plasma burner type concerned, such as a conically shaped discharge space with a cylindrical nozzle outlet. The outer shape of the nozzle is designed as a cone, formed with an approximately uniform wall thickness, which is dimensioned such that good stability of the nozzle and good conduction of the heat to the coolant is ensured. The nozzle is located in a nozzle holder. The nozzle holder consists of a corrosion-resistant material, such as brass, and has on the inside a centring mount for the nozzle and a groove for a rubber seal, which seals the discharge space against the coolant. In the nozzle holder, there are in addition bores offset by 180° for the coolant supply and return lines. On the outer diameter of the nozzle holder there is a groove for an O-ring for sealing the coolant chamber against the atmosphere and a thread and a centring mount for a nozzle cap. The nozzle cap, likewise made of corrosion-resistant material, such as brass, is shaped with an acute angle and has a wall thickness designed to make it suitable for dissipating radiant heat to the coolant. The smallest internal diameter is provided with an O-ring. For a coolant, it is simplest to use water. This arrangement is intended to facilitate the manufacture of the nozzles, whilst making sparing use of materials, and to make it possible to replace the nozzles quickly and also to swivel the plasma burner relative to the workpiece thanks to the acute-angled shape, thus enabling slanting cuts.
In the published patent application DE-OS 1 565 638 there is described a plasma burner, preferably for plasma arc cutting of materials and for welding edge preparation. The slender shape of the torch head is achieved by using a particularly acute-angled cutting nozzle, the internal and external angles of which are identical to one another and also identical to the internal and external angles of the nozzle cap. Between the nozzle cap and the cutting nozzle, a chamber is formed for coolant, in which the nozzle cap is provided with a collar, which establishes a metallic seal with the cutting nozzle, so that in this way a uniform annular gap is formed as the coolant chamber. The coolant, generally water, is supplied and removed via two slots in the nozzle holder arranged so as to be offset by 180° to one another.
In DE 25 25 939, a plasma arc torch, especially for cutting or welding, is described, in which the electrode holder and the nozzle body form an exchangeable unit. The external coolant supply is formed substantially by a coupling cap surrounding the nozzle body. The coolant flows through channels into an annular space formed by the nozzle body and the coupling cap.
DE 692 33 071 T2 relates to an electric arc plasma cutting apparatus. It describes an embodiment of a nozzle for a plasma arc cutting torch formed from a conductive material and having an outlet opening for a plasma gas jet and a hollow body section designed such that it has a generally conical thin-walled configuration which is slanted towards the outlet opening and has an enlarged head section formed integrally with the body section, the head section being solid, except for a central channel, which is aligned with the outlet opening and has a generally conical outer surface, which is also slanted towards the outlet opening and has a diameter adjacent to that of the neighbouring body section which exceeds the diameter of the body section, in order to form a cutback recess. The electric arc plasma cutting apparatus possesses a secondary gas cap. In addition, there is a water-cooled cap disposed between the nozzle and the secondary gas cap in order to form a water-cooled chamber for the external surface of the nozzle for a highly efficient cooler. The nozzle is characterised by a large head, which surrounds an outlet opening for the plasma jet, and a sharp undercut or recess to a conical body. This nozzle construction assists cooling of the nozzle.
In the plasma burners described above, the coolant is supplied to the nozzle via a water flow channel and removed from the nozzle via a water return channel. These channels are usually offset from one another by 180°, and the coolant is supposed to flow round the nozzle as uniformly as possible on the way from the supply line to the return line. Nevertheless, overheating is repeatedly found in the vicinity of the nozzle channel.
A different coolant flow for a burner, preferably a plasma burner, especially for plasma welding, plasma cutting, plasma fusion and plasma spraying purposes, which can withstand the high thermal loads in the nozzle and the cathode is described in DD 83890 B1. In this case, for cooling the nozzle, a cooling medium guide ring which can easily be inserted into and removed from the nozzle holding part is provided, which has a peripheral shaped groove to restrict the cooling medium flow to a thin layer no more than 3 mm thick along the outer nozzle wall. More than one, preferably two to four, coolant lines arranged in a star shape relative to the shaped groove and radially and symmetrically to the nozzle axis and in a star shape relative to the latter are provided at an angle of between 0 and 90° and lead into the shaped groove in such a way that they each have two cooling medium outlets next to them and each cooling medium outlet has two cooling medium inlets next to it.
This arrangement for its part suffers from the disadvantage that greater effort is required for the cooling, because of the use of an additional component, the cooling medium guide ring. Furthermore, the entire arrangement becomes bigger as a result.